As known, in many apparatus for manufacturing and handling security documents, such as banknotes, value sheets and the like, at least one inspecting station is provided to test the quality, genuineness and other features of such security documents.
The inspection is performed by grabbing the image of the document—which is suitably illuminated while passing through the inspecting station—processing it with more or less sophisticated analogical or digital methods and comparing it to previously generated and configured reference patterns.
The first basic feature required in order to obtain inspecting stations of suitable quality concerns the evenness of the security document-transporting movement.
So far, in known systems, the document to be inspected was made to advance past the inspecting device with a straightaway or curvilinear motion and with various techniques. However, the results obtained thus far have not been completely satisfying.
DE1953542 discloses an inspection system where an illuminator device is composed of fluorescent plates having difference fight lobes apt to shift the maximum brightness some distance away from the exit edge of said fluorescent plates. U.S. Pat. No. 5,498,879 discloses a device which provides uniform illumination in an apparatus for testing notes, in which the device is optically diffractive.
In order to understand the difficulty of the technical task undertaken herewith, it should be considered that, in the case of banknotes, for example, the document is a very thin (about 0.1 mm) 800×700 mm sheet that is transported at a speed of about 3 m/sec: this implies inevitable sheet vibrations and oscillations—especially generated by aerodynamic forces—which, if not correctly controlled, may completely compromise the reliability of the data acquired during the test.
Therefore, although prior art systems have reached acceptable quality standards, at the cost, in some cases, of adopting complex adjusting and control systems, there is still the need to reach a higher level of movement evenness, without compromising on the quality or excessively increasing the manufacturing difficulty and related costs.
Another feature, which, as may be easily guessed, affects the quality of the inspecting operation results, concerns the actual acquiring system.
Normally, the document image is acquired through electronic cameras; in particular line scan cameras performing a line-by-line scanning of each document.
Image acquisition may be performed basically using two techniques: by reflection or by transparency, depending on which security document feature is to be scanned and tested.
In a reflection system, the camera and the light source are placed on the same side relative to the document to be inspected, as shown in FIG. 1.
As it can be seen in the figure, a light source 1, through a wave-guide system (for example fibre optics) 2 and a collimation device 3, projects a bright band 1a focussed onto a document 4 to be inspected (also called target). On the same side of the lighting system there is also an acquisition camera 5, “reading” the light reflected by the document in order to transform it into an analogical or digital signal to be analysed with known techniques.
By varying the relative angle of the camera and of the lighting system relative to the document 4, it is possible to acquire images in different conditions, i.e. with specularly reflected light or with diffused light.
This arrangement is used to inspect features on the recto and verso of a document, such as a banknote.
In a transmitted light system, on the other hand, the camera and the lighting device are placed substantially opposite to each other relative to the document to be inspected, the latter being made to pass between the former. This arrangement is used to transparency-inspect specific features, such as watermarks, security threads and so on.
Some of the drawbacks afflicting the prior art systems are indicated below.
The cameras commonly used in this field are black and white or RGB line scan cameras, requiring a simple line of light, of sufficient intensity and quality, to be generated, in order to allow a correct acquisition. The quantity and quality of the light is a critical factor in this type of appliances, due to the high document transportation speed and thus the short exposure times: with a 0.25 ms/pixel resolution camera, at the above said typical speeds, the exposure time allowed is shorter than 25 ms/pixel line.
The line of light is generated by a suitable (halogen, arc, metal halide, etc.) lamp, which is directed towards a circular-shaped fibre optic input beam, by means of an elliptical reflecting mirror. At the opposite end of the beam, the fibres output is disposed so as to generate a line of light (for example, a 1.6×120 mm line). The light generated by the fibres diverges at a specific angle and is then collected by a system of lenses (usually cylindrical) and focussed onto the target within the camera field of vision.
The light that hits the target, however, if accurately measured with suitable instruments, is somewhat irregular and uneven, especially at the boundaries of the illuminated area.
In fact, it must be considered that each point in the field of vision receives the light from a certain portion of the illuminator (a segment measuring a few centimeters), because there is a certain angle of light source emission (in the case of a fibre optic endings, this angle is 60°). On the boundaries of the illuminated area the quantity of received light decreases rapidly, corrupting the uniformity of the luminous profile; if an illuminator were available which were much longer (ideally endless) than the usable length of the camera field of vision, this problem would not arise: however, this solution is not often applicable.
Beside these generic problems, transparency inspection suffers from another drawback. In fact, when no elements are interposed between the camera and the lighting system—which thing occurs, for instance, in the lapse between one document and the next, at the document edges or with partially transparent documents—the illuminator light hits the camera directly, causing a so-called blooming effect. The camera sensitive elements, in these conditions, exceed their saturation threshold and, for a short subsequent transient (depending on the response times typical of the acquiring electronics), the camera is no longer able to correctly acquire other images.
In order to solve the blooming problem, alternative arrangements have been proposed. For instance, GB2311130 teaches to arrange the optical axis of the camera at a certain angle relative to the illuminator axis. However, this arrangement does not allow reliable detection of potential holes in the document. In fact, the light, which normally focuses, onto the document surface, in the presence of a hole cannot be read by the camera, which reads a black background instead, not being able to tell a hole from a point of the document completely impermeable to the light radiation.
Thus, a first object of the invention is that of providing an inspecting system provided with a transportation device for thin security documents allowing excellent movement evenness.
A further object of the present invention is to provide an inspecting system provided with a lighting and image acquiring system that solves the prior art drawbacks, in particular having a homogeneous luminous profile, over the entire lighting field, and which, in the case of a transparency inspection arrangement, prevents camera blooming without the quality of the document anomalies detection being affected.